Since its discovery in 1953, REM sleep has been found to be almost universal among mammalian species. It is present in primates, ungulates, seals, carnivores, marsupials, insectivores, bats, rodents, cetaceans and other orders. The only animal with a well documented absence of REM sleep is the short-beaked echidna (Tachyglossus aculeatus). The short beaked echidna is one of only 3 existing species of monotreme. The other two monotreme species are the closely related long beaked echidna (Zaglossus bruijni) and the ancient duck billed platypus (Ornithorhynchus anatinus). Monotremes diverged from the line that led to placental and marsupial mammals 130 million years ago. The living monotremes have been shown to retain many characteristics of species ancestral to modern mammalian orders. They offer a unique opportunity to make inferences about the evolution of sleep. There has only been one study of echidna sleep, Allison et al.'s pioneering 1972 investigation. There have been no investigations of sleep in the platypus. We propose to determine whether the platypus has REM sleep. We will determine if the reported absence of REM sleep in the echidna is accompanied by a normal appearance of nonREM sleep activity patterns in brainstem neurons. We will monitor brainstem unit activity in the echidna to determine if any aspects of REM sleep are present in the cell groups known to change activity during this state. We will determine if locus coeruleus and raphe cells are silent during sleep or waking. We will determine if pontine units fire in a bursting pattern during sleep. We will map the distribution of cholinergic and aminergic cell groups in the brainstem of the echidna and platypus to determine how they differ from non-monotreme mammals. The Preliminary Data section demonstrates that we have overcome the technical and availability problems that have prevented the study of monotreme sleep. The function and origin of REM sleep remain the central questions of sleep research. REM sleep processes are likely to be involved in narcolepsy, the sudden infant death syndrome, depression, REM behavior disorder and other diseases. An understanding of how this state evolved would be of central importance to any comprehensive understanding of normal and pathological sleep state organization.